Blood pressure changes according to the body posture, it will be low when lying down and sitting and standing shows increase in pressure.
BP monitoring between a experienced person and an unexpereinced person may give very varied numbers
There is always some difference in BP in arms. The top number difference above 20 and lower number difference number below 10 should be considered significant.
It may be due to underlying phenomenon which may be very serious.
There is also a possibility of broken device, but it is always better to get a immediate opinion from a physician and preferrably a cardiologist.
You also have been prescribed aspirin
(when prescribed along metropolol) which is a given for prevention of stroke
, by preventing the blood vessel blockage.
So if you are in high risk category of heart disease